Method for mildly synthesizing strong ultraviolet intrinsic emission nano zinc oxide powder in one step

Abstract

The invention discloses a preparation method for mildly synthesizing strong ultraviolet emission zinc oxide nano powder in one step. In order to inhibit ZnO deep energy level defect luminescence and improve ZnO intrinsic emission intensity, citrate is selected as a complexing agent to form [Zn (C6H5O7)] <->, and the concentration of free Zn < 2 + > in the solution is reduced; the pH value of the solution is adjusted to be neutral (pH = 7-9), rapid reaction of Zn < 2 + > and OH <-> is inhibited, ZnO is enabled to slowly nucleate and grow in a mild reaction environment, the release rate and rapid growth of Zn < 2 + > are controlled to inhibit zinc gaps (Zni) and other defects, deep energy level defect luminescence is reduced, and the ultraviolet intrinsic luminous intensity of ZnO is remarkably improved. The ZnO nano-powder prepared by the method has good crystallinity and ultraviolet luminescence performance, can be applied to occasions with high time resolution and spatial resolution, and is expected to be applied to dynamic imaging. The preparation method has the advantages of simple preparation process, good stability, short period, environmental protection and the like.

Description

Technical Field

[0001] The present invention relates to a one-step mild method for synthesizing strong ultraviolet-emitting zinc oxide nanopowder. The method can prepare zinc oxide nanopowder with intrinsic emission by adjusting solution concentration, surfactant ratio and solution pH value, and belongs to the field of photoelectric functional materials. Background Art

[0002] A scintillator is a material that converts radiant energy into visible light or ultraviolet light. The most commonly used scintillators include zinc sulfide, CdWO4, sodium iodide, and BaF2. Since the 21st century, radiation detectors have rapidly developed in medical imaging, high-energy and nuclear physics, oil exploration, and other fields. Although conventional scintillators have made significant progress in scintillation efficiency, they still face challenges in temporal and spatial resolution. Temporal response and light yield are particularly important indicators for evaluating dynamic imaging quality, making the fluorescence decay time and light yield of the scintillator key factors in determining dynamic imaging quality.

[0003] ZnO is a direct bandgap semiconductor (3.37 eV) with a large exciton binding energy (60 meV). Its radiative exciton decay has a fast scintillation response (sub-nanosecond emission lifetime). Its high density (5.7 g / cm 3 ) and effective atomic number (28) ensure sufficient radiation cutoff capability. In addition, ZnO material has a high melting point (1975℃), is non-hygroscopic, chemically stable, and has strong radiation. Based on these advantages, zinc oxide with different structures has been used in various radiation detectors and has the potential for dynamic imaging applications. Zinc oxide has two main luminescence regions: one is ultraviolet intrinsic luminescence, whose luminescence wavelength is about 380nm and the luminescence decay time of ultraviolet luminescence is about sub-nanosecond; the other is deep energy level defect luminescence, whose luminescence wavelength is 430-800nm. The main deep defects include oxygen vacancies, zinc vacancies, oxygen interstitials, zinc interstitials, and oxygen anti-sites. The decay time of deep energy level defect luminescence is much slower than that of ultraviolet luminescence. Therefore, in applications that require fast response, the influence of deep energy level defect luminescence should be reduced.

[0004] Currently, zinc oxide materials used for radiation detection include single crystals, thin films, transparent ceramics, and nanostructures, prepared using chemical methods such as electrodeposition, hydrothermal methods, and sol-gel methods. Each preparation method has its own unique advantages. For example, coprecipitation offers the advantages of large-scale synthesis at room temperature, simple experimental equipment, easy impurity doping, and low cost. Magnetron sputtering offers advantages such as high deposition rate, high power efficiency, low substrate temperature, and minimal substrate damage. However, the photoluminescence and radioluminescence spectra, as well as the radioluminescence dynamics, of zinc oxide vary significantly with sample structure and preparation conditions, resulting in ultraviolet luminescence (~380 nm) and strong room-temperature defect-related luminescence (430-800 nm). Nanoscale zinc oxide materials can reduce self-absorption in zinc oxide, and nanostructured zinc oxide exhibits improved optical properties.

[0005] The main methods for preparing nanostructured ZnO include hydrothermal, chemical vapor transport, and vacuum thermal evaporation. The hydrothermal method's primary advantage is its ability to synthesize high-quality nanostructured ZnO at relatively low temperatures. To facilitate large-scale and low-cost production, the morphological parameters of the nanorods / wires can be controlled by adjusting the precursor concentration, reaction temperature, and solution pH. Furthermore, the type and concentration of dopant ions (such as Ga, In, and Mo) can be easily adjusted to produce more uniformly doped ZnO-based nanomaterials. Li et al. first grew a 100 nm ZnO seed layer on a quartz substrate by magnetron sputtering. They then added a mixture of HMTA, zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O], and hydrated gallium nitrate [Ga(NO₂)₃·xH₂O] in a certain ratio and hydrothermally heated at 95°C for 3 hours to obtain vertically grown ZnO:Ga nanorod arrays. Sinem et al. added sodium citrate and ammonia as precursor solutions to the growth solution and, using a buffer to control the reaction rate, obtained nanorod arrays that emitted ultraviolet light (~378 nm). Sun et al. synthesized twinned flower-like ZnO structures at a low temperature of 90°C via a one-step CTAB-assisted hydrothermal method, demonstrating excellent UV luminescence (~373 nm). Angubd et al. successfully fabricated ZnO nanorods preferentially aligned along the c-axis on crystalline silicon (Si) substrates using varying concentrations of hexamethylenetetramine [(CH2)6N4, HMTA] and zinc acetate dihydrate [Zn(CH3COO)2·2H2O, ZnAc] at a relatively low temperature of 85°C. Their optical properties exhibited strong picosecond ultraviolet (UV) emission.

[0006] In recent years, in order to suppress deep-level defect luminescence, researchers have optimized the ultraviolet luminescence performance of ZnO through synthesis methods and post-processing (annealing), atomic doping (interstitial hydrogen doping), application of various coating materials, and plasma enhancement, to obtain ZnO scintillators with better performance and improve luminescence efficiency, spatial resolution and stability.

[0007] To this end, the present invention has developed a one-step mild synthesis method for the preparation of strong ultraviolet emitting zinc oxide nanopowder. First, citrate can be used as a complexing agent to form [Zn(C6H5O7)] - , reducing the free Zn in the solution 2+ The concentration of Zn 2+ With OH - The rapid reaction allows ZnO to slowly nucleate and grow in a mild reaction environment, controlling the 2+ Release rate and rapid growth inhibition of zinc clearance (Zn i ) and other defects, thereby improving the intrinsic ultraviolet luminescence intensity of ZnO. Summary of the Invention

[0008] The present invention provides a one-step mild synthesis method for preparing zinc oxide nanopowder with strong ultraviolet emission. The preparation process of the present invention is as follows: zinc salt (Zn(NO3)2·6H2O or Zn(CH3COO)2·2H2O) is selected as the zinc raw material, citrate (Na3C6H5O7 or K3C6H5O7) is used as the complexing agent, and alkali salt (LiOH, KOH, NaOH) or ammonia (NH3·H20) is used as the pH adjuster. First, citric acid and zinc salt are dissolved to form [Zn(C6H5O7)] - , reducing the free Zn in the solution 2+ The solution was added to a hydrothermal reactor, heated at 150-200°C for 6-24 hours, and then cooled naturally to room temperature. The precipitate was collected by centrifugation and washed alternately with deionized water and ethanol for 3-5 times. The ZnO nanopowder sample was obtained by drying at 60-80°C for 6-12 hours.

[0009] The technical effect of the present invention is obvious. By adding an appropriate amount of citrate, its citric acid ion can act as a complexing agent with Zn 2+ Ions form stable [Zn(C6H5O7)] - Thus, the nucleation and growth rate of ZnO are slowed down. 2+ The complexation effect is equivalent to a "slow-release reservoir", gradually releasing Zn 2+Participate in the reaction, make the nucleation and growth process more uniform, and avoid uneven particles caused by explosive nucleation. At the same time, citrate ions can prevent ZnO particles from agglomerating through electrostatic repulsion or steric hindrance effects, reduce particle size distribution, and improve dispersibility. In addition, the complexing force of citrate ions is stronger under acidic conditions (pH < 7), so it is necessary to add alkaline substances to balance the hydrothermal reaction environment (pH = 7 ~ 9) so that ZnO can slowly nucleate and grow in a mild reaction environment. Therefore, the advantages of the present invention are twofold: First, it allows ZnO to slowly nucleate and grow in a mild reaction environment, avoiding zinc gaps (Zn i ) defects and incomplete crystallization, thereby ensuring the high crystallinity of ZnO nanopowders while obtaining good ultraviolet luminescence performance; secondly, the preparation process of the present invention is simple, the preparation process is stable, the cycle is short, and the yield is high, which can save production costs to a certain extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is the XRD pattern of the ZnO nanopowder sample prepared by the present invention.

[0011] Figure 2 This is a SEM image of the ZnO nanopowder sample prepared by the present invention.

[0012] Figure 3 This is a photoluminescence image of the ZnO nanopowder sample prepared by the present invention under 325nm ultraviolet light excitation. DETAILED DESCRIPTION

[0013] (1) The method for preparing a strong ultraviolet emitting ZnO nanopowder sample of the present invention is described in detail as follows:

[0014] Zn(NO3)2·6H2O, K3C6H5O7 and NaOH are used as raw materials, K3C6H5O7 is a complexing agent and NaOH is a solution pH regulator. 2+ The concentration is 0.05mol / L to 0.5mol / L, where the concentration of the complexing agent K3C6H5O7 is Zn 2+ :(C6H5O7) 3- The molar ratio is 10:1 to 10:3.5; the concentration of the pH adjuster NaOH is 0.5 g / ml to 1 g / ml.

[0015] (2) According to the preparation of 0.1 mol / L concentration of precursor, the corresponding amount of Zn(NO3)2·6H2O and K3C6H5O7 was weighed using a precision electronic balance. The weighed raw materials were placed in a 100 ml glass beaker, and 85% ethanol solution was added. Ultrasonic stirring was carried out for 30 min to dissolve the raw materials evenly.

[0016] (3) Slowly adding the pH regulator NaOH to the solution obtained in (2) dropwise, and monitoring the pH value of the solution in real time to control the final pH to 7;

[0017] (4) The solution obtained in (3) was stirred for 60 min to obtain a homogeneous system, and then the mixed system was transferred to a high-pressure reactor lined with polytetrafluoroethylene, kept at a constant temperature of 180°C for 12 h, and naturally cooled to room temperature;

[0018] (5) The reaction mixture obtained in (4) was centrifuged, the supernatant was removed, and the precipitate was washed three times with deionized water (the deionized water was preheated to 60°C), and then washed three times with anhydrous ethanol;

[0019] (6) The solid after centrifugation was placed in a constant temperature drying oven and dried for 12 hours, and the product was collected. The temperature of the constant temperature drying oven was 60°C.

[0020] Figure 1 The X-ray diffraction (XRD) patterns of the prepared ZnO nanopowder samples are plotted with the diffraction angle (2θ) in degrees on the abscissa and the diffraction intensity on the ordinate. The XRD patterns show reflections from the (100), (002), (101), (102), (110), (103), and (112) planes, confirming that all samples are polycrystalline and possess a hexagonal wurtzite crystal structure with a P63mc space group (JCPDS 36-1451).

[0021] Figure 2 The SEM image of the prepared ZnO nanopowder sample shows that the powder is anisotropic and irregular in shape, with a particle size of ~100 nm.

[0022] Figure 3 The photoluminescence image of the prepared ZnO nanopowder sample under 325nm ultraviolet light excitation shows strong ultraviolet emission under ultraviolet excitation.

Claims

1. A one-step mild synthesis method of strong ultraviolet emitting nano zinc oxide, the preparation process of the method is: first, citrate and Zn 2+ Form a stable soluble complex to inhibit Zn 2+ Direct hydrolysis of ZnO to control the nucleation and growth rate of ZnO; then add alkaline solution to the complex and adjust the pH value of the solution to 7-9 to ensure sufficient OH - , balance the hydrothermal reaction environment; finally, stir at room temperature for 60 minutes to fully mix. After completion, pour the solution into a hydrothermal reactor and react at 150-200°C for 6-24 hours to obtain strong ultraviolet emitting nano-ZnO with good crystallinity.

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